WO2011144460A1

WO2011144460A1 - Device and method for generating a wide band frequency comb

Info

Publication number: WO2011144460A1
Application number: PCT/EP2011/057244
Authority: WO
Inventors: Linda Mondin; François Lemaitre; Xavier Orlik
Original assignee: Centre National D'etudes Spatiales
Priority date: 2010-05-18
Filing date: 2011-05-05
Publication date: 2011-11-24
Also published as: FR2960312A1; FR2960312B1; EP2572239A1; EP2572239B1

Abstract

According to a first aspect, the invention relates to a device (1) for generating a continuous wide-band frequency comb, including: an optical fiber loop (3); a continuous laser source (2) providing a laser line at a reference frequency, the laser source being coupled to the loop so that the laser line provided by the source passes several times through the loop (3); a phase modulator (5) provided in the optical fiber loop, said phase modulator being capable of generating side lines from an incident central line, a plurality of evenly spaced lines being then generated over a wide band due to the passages thereof in the phase modulator; characterized in that it further comprises frequency transposition means (7) arranged in the optical fiber loop (3), said frequency transposition means (7) being adapted for offsetting said plurality of lines generated on the loop upon each passage in the loop.

Description

DEVICE AND METHOD FOR GENERATING A FREQUENCY COMB

BROADBAND

FIELD OF THE INVENTION

The invention relates to a device and a method for generating with a master laser a continuous broadband frequency comb formed of a plurality of regularly spaced lines, having the same spectral properties as the master laser and a reduced phase noise.

BACKGROUND OF THE INVENTION

Listed below are four preferred areas of use of a laser source having characteristics of frequency stability, low linewidth and the possibility of obtaining power at different wavelengths without impacting the first two parameters.

^ Frequency metrology, frequency and frequency characterization of laser sources;

^ Metrology of distances (need of a source at least bi-wavelength), either by flight time, or by interferometric measurement;

^ Cooling and manipulation of atoms (without the need to vary the frequency of a master laser with a significant gain on the duty factor; obtaining a power output by injection into a diode without degradation of the width of the ray);

Spectroscopy (possibility of using several wavelengths to probe a gas for example).

A solution often used to obtain broadband frequency references is to self-reference a frequency comb. This is done using an extra-short pulsed laser which provides very fine lines (kHz at best) spaced a few tens of MHz. Most of the current frequency combs are thus obtained using femtosecond pulse lasers (very wide band> 100nm), or using picosecond lasers which are developed for uses in which a lower number of spectral components are required.

In addition to the fact that the cost of such a pulsed laser is significant, the large number of lines provided (close in frequency) poses filtering problems and proves to be unsuitable for certain applications (measurement of distance, manipulation of atoms). Of the more the impulse regime raises questions for the use coupled with for example a fiber amplifier. In addition, in order to obtain power at a chosen wavelength, it is necessary to frequency lock a diode on a tooth of the comb fs. The frequency stability is then maintained, but the line width of the fs source is lost.

It is therefore preferable in the fields of application of the invention to use a continuous comb source insofar as a diode injection can be performed thus transferring to the power source not only the frequency stability of the comb source but also its intrinsic line width.

However, the rare frequency comb lasers developed in continuous mode use the longitudinal modes of a cavity (typically Fabry-Perrot cavity). The stability of the lines obtained will then depend on fluctuations in the length of the cavity.

Document EP 1 209 780 discloses a technique for generating a comb of lines regularly spaced, in continuous mode, over a broadband spectrum that does not use a cavity effect. This technique uses multi-passes in a fiber loop injected by a master laser, containing an electro-optical modulator and an optical amplifier.

The inventors have been able to demonstrate that by implementing this technique, the spectral finesse of the master laser is largely preserved, including in distant harmonics. However, the multi-passes of the master laser wave in the loop cause interference and make the spectrum unstable and not very usable.

This disadvantage is also mentioned in EP 1 209 780 in [0023], lines 22-24.

A solution to this interference problem is presented in EP 1 209 780. It consists in widening the laser source to allow the different passages in the loop to be decorrelated. But given the line width necessary to obtain this decorrelation (around 50 MHz, wider than the majority of power diodes), the comb generated has no application in metrology.

The aim of the invention is to remedy this drawback by proposing a technique that makes it possible to generate a comb of lines regularly spaced, in continuous mode, over a broadband spectrum (spread over several nanometers typically more than 10 nanometers) while maintaining the coherence of the signal and the phase relationships between the different harmonics (and over the entire range of the frequency comb), as well as a line width compatible with applications envisaged, without using an active control of the length of the loop (comb generation cavity).

STATEMENT OF THE INVENTION

For this purpose, the invention proposes, according to a first aspect, a device for generating a broadband frequency comb in a continuous mode, comprising:

an optical fiber loop,

a continuous-mode laser source delivering a laser line at a reference frequency, the laser source being coupled to the loop so that the laser line delivered by the source carries out multi-passes in the loop,

a phase modulator disposed in the optical fiber loop, said phase modulator being able to generate lateral lines from an incident central line, a plurality of regularly spaced lines being then generated over a wide band from their passages; in the phase modulator;

characterized in that it further comprises frequency transposition means arranged in the optical fiber loop, said frequency transposing means from shifting said plurality of lines generated on the loop each passage in the loop.

Some preferred, but not limiting, aspects of this device are:

the phase modulator and the frequency translation means are arranged in series on the loop;

the loop also comprises an optical amplifier;

the phase modulator consists of an electro-optical modulator;

it furthermore comprises a microwave synthesizer driving the phase modulator, said microwave synthesizer delivering a modulation frequency in the range 100 MHz to 50 GHz;

the means of frequency transposition consist of an acousto-optic modulator;

it furthermore comprises a radio frequency synthesizer controlling the frequency transposition means, said radio frequency synthesizer delivering a transposition frequency in the range 20 MHz to 400 MHz. According to a second aspect, the invention relates to a method of generating a broadband frequency comb in a continuous mode, in which a laser source is used which delivers a laser line at a reference frequency in a continuous regime, in which the optical fiber loop laser source in which is disposed a phase modulator adapted to generate side lines from an incident central line, and in which multi-pass of the laser line delivered by the laser source is performed in the optical fiber loop, a plurality of regularly spaced lines being then generated over a wide band from the multi-passes of the laser line in the loop, characterized in that one carries out a frequency transposition of said plurality of lines generated on the loop each time in the loop.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, objects and advantages of the present invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and with reference to the appended drawings in which: :

FIG. 1 is a diagram illustrating a possible embodiment of the device according to the first aspect of the invention

FIG. 2 represents a frequency comb obtained by the invention having a spectral range of the order of 2 nm optical;

FIG. 3a and FIG. 3b, respectively, represents a frequency comb having a spectral range of the order of 10 nm optical and extending generally to the right, respectively to the left, of the central line;

FIG. 4 is a diagram representing the bandwidth of a fiber amplifier

DETAILED DESCRIPTION OF THE INVENTION

Referring to Figure 1, there is shown a possible embodiment of a device for generating a broadband frequency comb 1 according to the first aspect of the invention.

The device 1 comprises an optical fiber loop 3 in which is disposed a phase modulator 5 capable of generating side lines from an incident central line. The phase modulator 5 is for example an electro-optical modulator controlled by a microwave synthesizer 6 delivering a high frequency modulation frequency Fmeo, typically in the range 100 MHz-50 GHz.

The device further comprises a laser source 2 outside the loop 3. The laser source 2 operates in a continuous mode and delivers a line of frequency Flm. The laser source is coupled to the loop 3 via an optical coupler 4 so that the laser line delivered by the source carries out multi-passes in the loop 3, a plurality of regularly spaced lines being then generated over a wide area. band from their passages in the phase modulator 5.

As discussed above, the multi-passes of the laser line in the loop comprising the phase modulator 5 make it possible to generate a comb of regularly spaced lines, in continuous mode, over a spectrum of several nanometers. The spectral finesse of the laser line delivered by the source 2 is also largely preserved, even in distant harmonics. However, the multi-passages of the wave in the loop cause interference that can make the spectrum unstable or unusable.

In order to maintain all the coherence of the signal and the phase relations between the different harmonics over the entire spectral range of the frequency comb, the invention provides for the insertion of frequency transposition means on the loop. . Thus, in the context of the invention, the loop 3 further comprises frequency transposition means 7 which offset said plurality of lines generated on the loop, at each passage in the loop. This frequency shift makes it possible to break the interference phenomenon while maintaining an excellent spectral range of the comb.

The frequency transposition means are for example constituted by an acousto-optical modulator 7 controlled by a radio frequency synthesizer 8 delivering a low frequency modulation frequency Fmao, typically in the range 20MHz-400MHz. The Fmao modulation frequency is in particular a discrete value among one of the following values: 20MHz, 40MHz, 80MHz, 160MHz, 200MHz, 250MHz and 400MHZ, and this according to the acousto-optical modulator used. It will be noted that each of these discrete values may be slightly adjusted. It is also possible to play on the harmonics of the basic modulation frequency of the acousto-optical modulator. It will be noted that the frequency translation means 7 are preferably arranged on the loop in series with the phase modulator 5.

The loop furthermore typically comprises a fiber amplifier 9 which amplifies the line comb provided by the series association of the phase modulator 5 and frequency translation means 7. The fiber amplifier is for example of the EDFA type ("Erbium -Ytterbium Doped Fiber Amplifier "designating an erbium-doped optical fiber amplifier).

FIG. 1 also shows an optical spectrum analyzer 10 coupled to the loop 3 for analyzing the frequency comb obtained by the multi-passes in the loop. It is with the aid of such an analyzer 10 that the spectra of FIGS. 2 and 3a-3b have been obtained.

It will be understood that in the context of the invention, the frequency comb is obtained by multiple passages in the electro-optical modulator (MEO in the following) high frequency fiber bundle. This type of component (for example a fibered Mach-Zender) can effectively modulate at a very high frequency (20GHz for example) a laser beam with a phase shift voltage (VTT) of the order of ten V. It is therefore possible with this type of component to create very wide frequency sidebands with contained voltages.

Thus, the combination of the laser beam delivered by the source 2 and the output of the fiber amplifier 5 is modulated in the MEO according to the frequency Fmeo, transposed in the acousto-optical modulator 7 (MAO in the following) of the frequency Fmao, then reinjected into the amplifier 9. At each cycle, a sinusoidal phase modulation is added to the existing signal, as well as a transposition. The harmonic lines thus generated at each cycle do not interfere, thus avoiding the problems of interference observed without the presence of MAO.

In theory, interference could emerge after a large number of cycles if the Fmeo modulation frequency was an exact multiple of the Fmao transposition frequency. However, in practice, given the large difference between the modulation frequency Fmeo (typically of the order of several GHz) and the Fmao transposition frequency (a few tens of MHz), it is difficult to end up in an unstable configuration . The laser source used in the context of the invention is preferably a laser source having interesting spectral properties in terms of intrinsic stability of frequency, power and linewidth.

In particular, a fiber laser source close to 1560 nm can be chosen. Its linewidth is of the order of ten kHz. The choice to work in the telecom wavelengths is motivated by the existence of a large number of low-cost fiber components with the qualifications required by the Telcordia standard (analogue of the old MIL standard).

Moreover, the frequencies close to 1560nm make it possible to benefit from the close transitions of Rubidium @ 780nm (by doubling of frequency) or those of acetylene, in the band 1530-1560 nm, for the stabilization in frequency of the source at a level several 10 ^{~ 12} / VHz.

It will however be understood that the technique of obtaining the comb is completely transferable to any other wavelength for which a laser source having interesting spectral properties and a narrow frequency reference is available. Electro-optical and acousto-optical components are available for various spectral domains while amplifiers can be developed with materials other than Erbium.

In the context of the invention, the frequencies Fp of the lines of the frequency comb are calculated as follows: Fp = Flm + k.Fmeo + n.Fmao, with k £ Z n £ N and where:

Flm denotes the frequency of the laser line delivered by the source laser;

the factor k comes from the sinusoidal phase modulation at the frequency Fmeo which generates multiple harmonics whose levels depend on the modulation index;

- the factor n comes from the multi-passages in the loop: at each cycle the whole spectrum is transposed from Fmao.

It is understood by this formula that the lines of the comb generated by the device according to the first aspect of the invention are tunable.

Furthermore, it should be noted that contrary to existing techniques that generate lines in continuous mode using the longitudinal modes of a cavity, the device proposed by the invention generates these lines without using the boundary conditions of a cavity and therefore without be subject to fluctuations in length thereof. On the contrary, the proposed technique uses the excellent stability of radio frequency sources to generate the gap between the different lines, which explains their much better stability, without any slaving.

Thus, from a reference laser frequency, it is possible to generate lines with the same spectral properties over more than 10 nm optical, which represents a significant advance over known techniques at best, without slaving. active cavity length, a continuous and coherent comb spreading its lines about a few nm.

FIGS. 2, 3a and 3b show different frequency combs generated by the device according to the first aspect of the invention. The comb of FIG. 2 has a minimum spectral extension of the order of 2 nm optical, while the comb of FIG. 3a and FIG. 3b respectively has a maximum spectral extension of the order of 10 nm optical and s' extending generally to the right, respectively to the left, of the central line.

It should be noted that in the context of the invention, the factor limiting the spectral range of the comb appears to be the bandwidth of the fiber amplifier shown in FIG. 4. The development of broadband amplifiers should thus make it possible to obtain a greater spectral coverage. Insofar as the amplification is shared over all the injected spectral components, a larger spectral band will have to be accompanied by a larger amplification to maintain the same level of power per tooth of the comb.

It will finally be understood that the invention is not limited to a device according to its first aspect, but also extends to a method for generating a broadband frequency comb in a continuous mode, in which a laser source is used. providing a laser line at a reference frequency in continuous mode, in which the laser source is coupled to an optical fiber loop in which a phase modulator is arranged, capable of generating side lines from an incident central line, and in which the laser line provided by the laser source is multi-pass through the optical fiber loop, a plurality of regularly spaced lines being then generated over a wide band from the multi-passes of the laser line in the loop. , characterized in that a frequency transposition of said plurality of lines generated on the loop is performed at each passage in the loop.

Claims

1. Device (1) for generating a broadband frequency comb in a continuous mode, comprising:

an optical fiber loop (3),

a laser source (2) in continuous mode delivering a laser line at a reference frequency, the laser source being coupled to the loop so that the laser line delivered by the source carries out multi-passes in the loop (3),

a phase modulator (5) arranged in the optical fiber loop, said phase modulator being able to generate side lines from an incident central line, a plurality of regularly spaced lines being then generated over a wide band of share their passages in the phase modulator,

characterized in that it further comprises frequency translation means (7) arranged in the optical fiber loop (3), said frequency translation means (7) shifting said plurality of lines generated on the loop at each passage in the loop.

2. Device according to claim 1, wherein the phase modulator (5) and the frequency transposition means (7) are arranged in series on the loop.

3. Device according to one of the preceding claims, wherein the loop also comprises an optical amplifier (9).

4. Device according to one of the preceding claims, wherein the phase modulator (5) is constituted by an electro-optical modulator.

5. Device according to one of the preceding claims, further comprising a microwave synthesizer (6) driving the phase modulator (5), said hyper frequency synthesizer delivering a modulation frequency in the range 100 MHz-50 GHz.

6. Device according to one of the preceding claims, wherein the frequency transposition means (7) are constituted by an acousto-optic modulator.

7. Device according to one of the preceding claims, further comprising a radio frequency synthesizer (8) driving the frequency transposition means (7), said radio frequency synthesizer delivering a transposition frequency in the range 20 MHz-400 MHz.

8. A method of generating a wide-band frequency comb in a continuous mode, in which a laser source (2) delivering a laser line at a reference frequency in continuous mode, in which the laser source (2) is coupled (2) an optical fiber loop (3) in which is disposed a phase modulator (5) capable of generating side lines from an incident central line, and in which multi-pass of the laser line delivered by the laser source (2) in the optical fiber loop (3), a plurality of regularly spaced lines being then generated over a wide band from the multi-passes of the laser line in the loop, characterized in that one performs a frequency transposition of said plurality of lines generated on the loop at each passage in the loop.